Apparatus for measuring capacitance of a low value three-terminal capacitor with a resonance technique

The unknown capacitor or a standard capacitor are switched, one at a time, in parallel to a frequency setting circuit which sets the frequency of operation. The parallel combination of the unknown capacitor and the frequency setting circuit form a first parallel resonant circuit and the parallel combination of the standard capacitor and the frequency setting circuit form a second resonant parallel circuit. These resonant circuits are part of the feedback network of an oscillator. The bottom ends of the resonant circuits are connected to an instrumentation amplifier for maintaining the bottom ends at vertual ground in order to make three terminal measurements. A frequency meter is provided for measuring the frequency of resonance of the first resonant circuit and the second resonant circuit.